The control and/or elimination of undesirable impurities and by-products from various manufacturing operations have gained considerable importance in view of the potential pollution such impurities and by-products may generate. One conventional approach for eliminating or at least reducing these pollutants is by oxidizing them via incineration. Incineration occurs when contaminated air containing sufficient oxygen is heated to a temperature high enough and for a sufficient length of time to convert the undesired compounds into harmless gases such as carbon dioxide and water vapor.
In view of the high cost of the fuel necessary to generate the required heat for incineration, it is advantageous to recover as much of the heat as possible. To that end, U.S. Pat. No. 3,870,474 (the disclosure of which is herein incorporated by reference) discloses a thermal regenerative oxidizer comprising three regenerators, two of which are in operation at any given time while the third receives a small purge of purified air to force out any untreated or contaminated air therefrom and discharges it into a combustion chamber where the contaminants are oxidized. Upon completion of a first cycle, the flow of contaminated air is reversed through the regenerator from which the purified air was previously discharged, in order to preheat the contaminated air during passage through the regenerator prior to its introduction into the combustion chamber. In this way, heat recovery is achieved.
U.S. Pat. No. 4,302,426 discloses a thermal regeneration anti-pollution system which adjusts for excessive temperatures in the high temperature incineration or combustion zone. To that end, the temperature in the combustion zone is sensed, and when a predetermined high temperature is reached therein, the gases that normally would be passed through the heat exchange bed are instead bypassed around the bed, then combined with other gases that have already been cooled as a result of their normal passage through a heat exchange bed, and are exhausted to atmosphere.
Regenerative thermal oxidation is used when the concentration of the volatile organic compounds (VOC'S), such as combustible solvents or fuels, in polluted process gases lie outside the limits of the explosive levels of the VOC's in the gas being processed. If, at the same time, the concentration of the VOC's, also referred to as the energy density parameter, is below the self-sustaining margin to maintain their thermal oxidation, a burner or other heating device may provide the supplemental energy. To heat the polluted process gas, the sensible energy content of the oxidized (i.e., cleaned) process gas can mostly be consumed. Therefore, two main advantages are obtained by eliminating the burner operation or other heating device: the energy efficiency of the system increases since no combustion air necessary to operate the burner needs to be added and heated; and the potential for generation of noxious gases (NOx), such as those that may be formed inside a burner flame, is decreased or eliminated.
Usually, by measurement of the temperature between regenerative thermal oxidizer heat exchange beds and/or inside the heat exchanger beds, and further by comparison of the sensed temperature(s) with fixed set point(s), the required supplemental fuel is detected. Then, according to the temperature difference(s), the control adjusts for the injection rate of the necessary fuel into the system to increase the energy density of the air stream to the oxidizer.
This approach is feasible as long as the exothermic energy of the supplemental fuel is solely employed to sustain the thermal oxidation. However, the methods needs to be refined if more fuel is introduced than needed to maintain the thermal oxidation without the use of a supplemental heat source such as a burner. This may occur, for example, where excess fuel is injected for branching off high caloric enthalpy streams in order to use their energy in other processes, such as a second heat exchanger of as a heat source for a dryer.
The reason for modification is that heat exchanger media beds typically contain ceramic or other media characterized by a high specific heat capacity. Therefore, the heat exchanger beds are able to store an ample amount of energy and may transfer temporarily more heat to the process gas than received back. This imbalance can take place without detection, since no temperature alteration initially occurs. Nevertheless, the delayed temperature change triggers an adjustment of the fuel injection. The temperature may then rise or not rise, depending on the severity of the energy imbalance. In the worst case, a phenomenon of collapsing temperature profile occurs, wherein the control for the fuel injection cannot compensate for the heat imbalance since the duration of the oxidation of the fuel (i.e., residence time) increases with decreasing temperatures and the chemically bound energy of the fuel may remain partly unreleased.
In order to prevent collapsing temperature profiles in the heat exchanger, an improved and fast-acting fuel control is desired.